1. Field of the Invention
The present invention relates to a method for direct measurement of fluorescent products in polycarbonate compositions and, more specifically, to a method for rapid direct quantification of thermal Fries product in the products of melt polycarbonate reactions.
2. Discussion of Related Art
Conventional industrial plants synthesize polycarbonate by mixing together an aqueous solution of a dihydroxy compound (e.g., bisphenol-A) with an organic solvent (e.g., dicloromethane) containing a carbonyl halide (e.g., phosgene). Upon mixing the immiscible organic and aqueous phases, the dihydroxy compound reacts with the carbonyl halide at the phase interface. Typically, a phase transfer catalyst, such as a tertiary amine, is added to the aqueous phase to enhance the reaction. This synthesis method is commonly known as the "interfacial" synthesis method for preparing polycarbonates, and the product of this synthesis is commonly known as LF grade polycarbonate.
The interfacial method for making polycarbonate has several inherent disadvantages. First, it can be a disadvantage to operate a process that requires phosgene as a reactant. Second, the process utilizes large amounts of an organic solvent, which can require expensive precautionary measures to prevent deleterious environmental effects. Third, the interfacial method requires significant capital investment in equipment. Fourth, polycarbonate produced by the interfacial process is prone to exhibiting inconsistent color, high levels of particulates, and high chloride concentration.
An alternate polycarbonate manufacturing method has been developed which avoids several of the aforementioned problems. This synthesis technique, commonly referred to as the "melt" process, involves the transesterification of a carbonate diester (e.g., diphenylcarbonate) with a dihydroxy compound (e.g., bisphenol-A). This reaction is typically performed without a solvent and is driven to completion by mixing the reactants under reduced pressure and high temperature with simultaneous distillation of the phenol produced by the reaction. Polycarbonate produced by the melt process is typically referred to as LX grade polycarbonate. The melt process provides many advantages over the interfacial process. More specifically, the melt process does not employ phosgene; it does not require a solvent; and it uses less equipment. Moreover, the polycarbonate produced by the melt process does not contain chlorine contamination from the reactants; it has lower particulate levels; and it has a more consistent color. Therefore, in certain circumstances, it can be highly desirable to use the melt process in production facilities.
However, the melt process tends to produce polycarbonate with significantly higher level of branching than that produced by the interfacial process. This branching is the result of a side reaction called the Fries rearrangement, which involves the conversion of a phenolic ester into corresponding ortho and para hydroxyketones. The rearrangement is based on the Fries rule, which postulates that the most stable form of a polynuclear compound is that arrangement which has the maximum number of rings in the benzenoid form.
The Fries rearrangement product in polycarbonate is typically the result of exposure to elevated temperatures in the presence of an active catalyst. The primary Fries product is a salicylate ester that, under melt polymerization conditions, can further react to form a tri-functional molecule that acts as a branch point for the resulting polymer. In this context, the generation of the Fries branch point structure can lead to polymer branching, thereby generating inconsistent melt behavior. In various applications, this branching significantly increases the ductility of the polycarbonate and is, therefore, undesirable.
As the demand for high performance materials has continued to grow, new and improved methods of providing improved products more economically are needed to supply the market. Due in part to the advantages inherent in polycarbonate production by the melt process, there is significant interest among industry members in producing LX grade polycarbonate with low Fries product content. In this context, various reactant and catalyst combinations for melt polymerization are constantly being evaluated; however, the identities of chemically or economically superior reactant systems for melt polymerization processes continue to elude the industry. As parallel screening gains popularity in all areas of chemistry, high-throughput screening of potential reactant systems will become increasingly important. As such, new and improved methods are needed for rapid, direct quantification of reaction products.